A superconducting strand is generally constituted by:
one or more filaments (up to several hundreds of thousands) of superconducting material; and
a matrix of "normal" metal in which the superconducting filaments are embedded.
This matrix is generally made of copper or aluminum and it serves several functions:
stabilizing the superconductor: to do this, the material of the matrix must have increasing thermal and electrical conductivity for increasing filament size; and
providing mechanical strength to the strand when the strand is subjected to electromagnetic forces.
The forming of new high critical temperature superconducting materials having a high critical temperature into wires poses special problems. These materials are oxides, e.g. YBa.sub.2 Cu.sub.3 O.sub.7-y, and they are therefore not deformable. They can be formed only when in the powder state, as is true for other superconductors, for example compounds of the "Chevrel phase" type described in French patent No. 2,553,565, dated Oct. 18, 1983.
In addition, the superconducting properties of these oxides are highly sensitive to their environment, in particular their oxygen environment, and the sintering of the powder which is required in order to provide electrical continuity between the grains must be performed in an oxidizing atmosphere. If the powder is enclosed in a metal sheath, it is not possible to provide the required oxygen environment.
Further, it is not possible to use copper as the matrix material since in the oxide state it reacts with the superconducting oxide to form phases which are more stable but not superconducting. For example, when using YBa.sub.2 Cu.sub.3 O.sub.7-y, the decomposition products are Y.sub.2 BaCuO.sub.5 and BaCu.sub.2 O.
Two solutions have already been proposed to these problems.
In the first case, a filament of superconducting oxide is made in a matrix of copper or copper alloy (e.g. CuNi), and then after the sintering heat treatment, the copper matrix is dissolved so as to leave the oxide filaments in the presence of oxygen.
Heretofore, the matrix has been dissolved in order to seek maximum current transport capacity in the superconducting filament. Although this method has been tested on short lengths of filament, it is not suitable for being industrialized. Firstly, the absence of copper means that the functions of thermodynamic stabilization and of mechanical strength for the filament are no longer provided. Secondly, industrial manufacture would require long lengths of copper to be dissolved and this would give rise to problems in maintaining the mechanical integrity of the superconducting material prior to sintering.
The second case has been investigated more thoroughly, and it uses a matrix made of silver. In addition to its high cost, silver is not mechanically strong; further, it appears that silver degrades the superconducting properties of the filament. This solution is therefore not suitable for major industrial development.
The object of the present invention is to provide a composite superconducting strand having a high critical temperature and whose structure is such as to be capable of being manufactured industrially.